Overview
This lecture covered the microscopic structure of bone tissue, focusing on the composition and function of bone matrix, types of bone cells, and the organization and purpose of compact and spongy bone.
Bone Matrix Composition
- Bone is a connective tissue with cells embedded in a hard extracellular matrix.
- The inorganic matrix makes up most of the bone's weight and consists mainly of calcium and phosphate in the form of hydroxyapatite, providing strength and resistance to compression.
- The organic matrix (about 35% of bone’s weight) contains collagen fibers, proteoglycans, glycosaminoglycans, and glycoproteins.
- Collagen fibers give bone tensile strength and resistance to pulling and twisting forces.
- The organic matrix components attract water, helping bones resist compression and maintain flexibility.
- Both mineral and organic components are essential for proper bone function; deficiency in either leads to brittle or overly flexible bones.
Types of Bone Cells
- Osteogenic cells are stem cells that differentiate into osteoblasts.
- Osteoblasts are bone-building cells that secrete the organic matrix.
- Osteocytes are mature bone cells that maintain the bone matrix and reside in lacunae.
- Osteoclasts break down bone matrix, releasing minerals into the blood for use elsewhere in the body.
Bone Remodeling and Regulation
- Bone remodeling is a dynamic balance of building and breaking down bone tissue.
- After growth is completed, bone building and breakdown are generally balanced.
- Hormones such as estrogen inhibit osteoclast activity and loss of estrogen increases bone breakdown, raising the risk of osteoporosis, especially in women.
- Bone acts as a storage site for calcium, which can be released into the blood as needed.
Compact Bone: Structure and Function
- Compact bone forms the dense outer layer (cortex) of bones.
- The functional unit is the osteon (Haversian system), consisting of concentric lamellae arranged around a central canal.
- Osteons are tightly packed columns providing strength against compression and trauma.
- Osteocytes reside in lacunae between lamellae and are connected via canaliculi for nutrient exchange.
- Perforating (Volkmann's) canals bring blood vessels horizontally into bone, which then connect to the central canals.
- Circumferential lamellae line the outer and inner surfaces of the bone, while interstitial lamellae fill gaps between osteons.
- Weight-bearing exercise stimulates bone formation, while lack of use leads to bone loss (e.g., in astronauts).
Spongy Bone: Structure and Function
- Spongy bone lies inside compact bone and has a porous, lattice-like structure made of trabeculae.
- Trabeculae are layers of bone arranged to resist forces from multiple directions without adding excess weight.
- Spongy bone provides structural support and houses red marrow, which produces blood cells.
- No osteons or central canals are present; osteocytes get nutrients from marrow spaces.
- Bone architecture adapts to mechanical stress; more trabeculae form where forces are greatest.
Key Terms & Definitions
- Bone Matrix — The extracellular material of bone, composed of inorganic minerals and organic proteins.
- Osteon (Haversian system) — The structural unit of compact bone, made of concentric lamellae around a central canal.
- Lamellae — Layers of bone matrix within compact or spongy bone.
- Lacunae — Small spaces housing osteocytes in bone tissue.
- Canaliculi — Small channels connecting lacunae for nutrient and signal exchange.
- Trabeculae — Lattice-like structures in spongy bone supporting bone marrow.
- Osteogenic cells — Bone stem cells that generate osteoblasts.
- Osteoblasts — Cells that build new bone matrix.
- Osteocytes — Mature bone cells that maintain the existing matrix.
- Osteoclasts — Cells that break down bone matrix for remodeling and mineral release.
- Hydroxyapatite — Mineral form of calcium phosphate providing bone hardness.
- Collagen fibers — Protein fibers offering tensile strength to bone.
Action Items / Next Steps
- Prepare to discuss the functions of compact bone vs. spongy bone in class.
- Review the cellular and matrix components of bone.
- Next lecture will cover bone formation processes.